What is Personal Rapid Transit (PRT)?

PRT is a transportation concept that has been developing over the past sixty years.  PRT vehicles are small two to six person (usually four person) modules that operate independently, automatically, and intelligently on some type of dedicated guideway.  The guideway is located on an exclusive right of way.  Guideways can be paved surfaces, rails, monorails, or similar schemes.  In some designs power for the vehicles is drawn from the guideways.  Other designs are powered by batteries that can be charged when the vehicles are at a station; this is called opportunity charging.  Many designs use vehicles that are rubber tired so they are quiet and unobtrusive.  Guidance can be obtained in different ways such as induction from buried wires, sensors, or optical means or mechanically from the guideways.  PRT vehicles operate on demand 24/7.  Each ride is individual and stops are not made at intermediate stations; the ultimate destination is selected by the passenger(s).  This means that PRT stations are small sidings off of the line.  PRT can utilize a network of interconnected guideways and or a linear layout.  This characteristic makes a PRT system easily expandable.

Because of the small passenger-to-vehicle weight ratio and the independent on demand operation, PRT is the most sustainable of all present powered transportation options.  How many times have you seen an empty bus or dinky operating in Princeton?

PRT vehicles are nimble, have a tight turning radius, and can quickly change elevation.  This means that their right of way has fewer constraints than conventional transportation modes because it can be 3 dimensional.  In addition, rights of way can be obtained even in overbuilt areas.

Worldwide, a number of firms are developing PRT systems.  To date no system is operational in a typical urban or small town setting, despite several attempts; several new efforts have now been started.  Past failures came mainly from political, project management, and cost issues and not from major problems with the underlying technologies.  The sponsoring entities were incapable of a long-term effort.  An entity such as Princeton University would be strong enough and smart enough to succeed. The technologies used for PRT are field-proven in other applications. 

A pioneering system best described as Group Rapid Transit has been operational in Morgantown, WV since 1975 and is currently being considered for upgrades.  The first link of a true PRT system has been completed at Heathrow airport in the UK.  The Heathrow system has provided over 300,000 rides with 99.8% uptime in the first year.  Additionally, another system is operational in Masdar City, Abu Dhabi in the UAE.  A major system has just gone under contract in Amritsar, Punjab, India and another is being built in South Korea.  An airport connector system is moving forward in San Jose, CA.  In Sweden, over 60 cities have planned for future PRT.  There are groups in Mountain View, Santa Cruz, Minneapolis/St. Paul, Austin, Cincinnati, Saint Louis, Atlanta, Seattle, Ithaca, and in other places worldwide that are actively considering PRT.

PRT is a viable choice for internal university transit even without public ridership.

Learn More About PRT:

An excellent summary of PRT can be found on the "Personal Rapid Transit" page of Wikipedia.

NJ Transit and NJ DOT funded a report "The Viability of Personal Rapid Transit in New Jersey" in 2007.  It is a thorough and comprehensive study.  What can be gleaned from its conclusions is that Princeton would be an excellent site for a pilot demonstration PRT system, and that building such a system would be good for New Jersey.

Other PRT resources:

For more information on the Heathrow and Bath Projects please see:

Links to local newspaper articles about SPURTS:

On 25 Sept 2010, Princeton Future hosted a forum where supporters of different transportation options presented their views.  The options discussed were light rail, bus rapid transit, keeping the dinky, and PRT.  The case for PRT was made by Ed Anderson and Chip Crider.  Some records of this forum are available at princetonfuture.org, including the slide show for the SPURTS presentation.  Also at princetonfuture.org is an audio recording of the two PRT presentations.  Ed Anderson's presentation begins at 1:41 and Chip Crider's begins at 2:03.

Rationale and Background Discussion:

We are about to see some major societal changes caused by permanent higher energy cost, uncontrolled growth, our culture of consumption and depleted resources.  Many of these changes will be in the systems we use to get around.  At present we are unprepared.

Projections state a 55% traffic increase in Princeton by 2020; that's only 8 years away.  This will mean our town streets will be gridlocked and/or congested for several periods each day.  All present and planned systems depend on public streets either totally or for the last mile.  This means shuttles, regular busses, jitneys, autos and the Bus Rapid Transit (BRT) will all be compromised at the times we need them the most.  If expensive gas shifts the 55% traffic increase date farther into the future because people are using public transit instead - then this causes earlier increased demand for public transit (we've already seen this happen).  If the projected 55% traffic increase date is too far out then the increased public transit demand will also come earlier.  Either way, public transit that includes the "last mile" must happen; it is our only hope of getting rid of that second car.

For Public Transit to Work, it must:

Why Should Princeton University be Interested in Transit:

Town, Gown, and Our Governments

System Requirements:

Proposed Plan for the Initial System:

The initial system is what the university should build as a demonstration system to serve their needs, the towns' needs, and to provide numerous research and development opportunities.  The proposed plan is shown here: SPURTS Pilot Plan

Proposed Downtown Joseph Henry Station Plan for the Initial System:

The Joseph Henry Station is integral to the initial system: Joseph Henry Station (PDF):

The 30 Year Plan:

The 30 Year Plan (PDF) shows how the system might evolve over 30 years.  By this time the system might be operated by some sort of local authority or public/university partnership.

You will note that no PRT service has been detailed for the interior of the campus. This is simply because the university has a far better knowledge of their transportation requirements in that area. PRT would be excellent in that area.

Everywhere a grade level or elevated SPURTS line is proposed a parallel bike path could and should be made.

How the SPURTS Project Might Unfold:


For more detail on costs see our most recent page (April, 2012): How much will it cost?

What the University Should Do:

Comparing Transit Options Is Not Easy:

There are so many forces touting different transportation options that it is difficult for the non-expert to make an informed conclusion.  For instance, some options, like bus, non-rail streetcars and of course automobiles do not require a dedicated guideway; they operate on the pubic streets.  Their guideway is not free; it is simply paid for by someone else and in another way.  In the US this is usually through government funding and the gasoline tax.  One has to compare apples and apples.  Today every salesman knows that you paint your product green and claim that it is sustainable.  There is a huge amount of that behavior in the transportation industry.  The simplest way to compare options is to look at the energy consumed per passenger mile.  The US DOE has been collecting aggregate transportation statistics for 30 year and releases them yearly in "The Transportation Energy Data Book".  For example, the average energy usage for heavy rail is 2250 BTU/passenger mile, for light rail it is 3526 BTU/passenger mile and for the personal automobile it is 3500 BTU/passenger mile.  Busses average about 4500 BTU/passenger mile.  According to Ed Anderson PRT energy consumption is 1880 BTU/passenger mile, which is about the same as that for a motorcycle.  Heathrow estimates their energy consumption at about 839 BTU per passenger mile.  That is why PRT is our best hope for the future considering shrinking energy supplies and higher energy costs.

There are 2 factors that influence these numbers.  One is the ratio of passenger weight to vehicle weight; PRT is best in that comparison.  For instance, the dinky car weighs 140,000 lbs. and can carry 117 passengers seated and an estimated 25 standees per car.  Using the Coast Guard standard average passenger weight of 185 lbs. the ratio is 1 to 6.5.  Using a composite weight for a PRT vehicle of 1600 lbs. and a seating capacity of 4 the ratio is 1 to 2.2.  It takes energy to haul all that extra weight around.  Light rail ratios are similar to those for heavy rail but can easily vary.  The "light" in light rail does not refer to the vehicle weight; it refers to the vehicle design and operating characteristics.  In fact, some light rail vehicles are even heavier than heavy rail vehicles.  If the light rail vehicle has batteries for brief operation without catenary power the ratio is even worse.  In summary, the dinky vehicle weight per passenger is 3.1 times the PRT vehicle weight per passenger.

The second variable is the load factor.  That is simply the average passenger occupancy achieved as compared to the maximum possible occupancy.  Unfortunately, there are no load factors reported simultaneously with the DOE data.  Poor load factor is one reason why some light rail systems have been reported to use up to 9 times the energy per passenger mile than the heavy rail average.  Small light rail systems in smaller population centers are generally the worst performers.  PRT has the advantage that it can achieve higher load factors both because it has a smaller vehicle passenger capacity and it runs on demand vs. scheduled.  Further, field experience has shown that the public will often share PRT vehicles simply because it is environmentally correct choice.  The beauty of PRT is that the load factor for a 4 person vehicle can never be below .25 (1 passenger where there could be 4) because if there are no passengers the trip just is not made.  The load factor for the Heathrow PRT is about .75 at peak times and .33 off peak.

Does the dinky achieve an energy cost per passenger mile equal to or better than the national average for heavy rail?  This is highly unlikely for 2 reasons.  The first is the load factor.  The dinky is packed in the AM rush and the PM return home and pretty empty (and often totally empty) the rest of the time.  Weekends are variable also. So the dinky load factor is probably worse, lots worse.  The second reason is that a lot more energy is used starting and stopping than in running at constant speed.  The dinky does a lot of starting and stopping in comparison to the distance traveled; the run is only 2.7 miles long.  So the claim that the dinky is sustainable is unfounded from an energy standpoint.

The dinky load factor can be calculated from the data given out in mid 2011 by NJ transit officials at a boro meeting.  According to the posted dinky schedule, the dinky makes about 23,368 total trips per year (not round trips).  It consists of 2 cars for redundancy (according to NJT).  Each car seats 117 so that gives 234 possible seated passengers and 5,468,346 possible rides per year.  Currently passengers make 355,00 one way trips per year and that has been essentially unchanged for the past 10 years.  Thus, the load factor is .065 for seated passengers; if you consider standees also the number is even worse.  This means that only 1 in every 16 possible rides is used; the other 15 are wasted.  Parenthetically, if the dinky operators were allowed to uncouple one of the cars during off peak times (and hence lose redundancy) the load factor could be almost doubled.  Work rules do not permit this.  In either case, the sustainability claim really does not hold up here!

Another way to compare options is operating cost and, once again, it gets complicated.  There is the cost of the operator for most other options; with PRT the passenger is the operator so that is a big savings.  There are supervisory and control costs with all options.  Then there are depreciation costs for the capital equipment.  There are usually costs for guideway maintenance.  Every option has vehicle maintenance costs and the cost of that maintenance facility itself.  With PRT the facility size and required equipment are on par with auto repair facilities.  For heavy rail and light rail the requirements are much more daunting.  The facility needs to be large and have lots of floor space to work, rebuild, and store parts simply because the vehicles are so much bigger.  Heavy duty lifting and material handling equipment is also needed.  The machinery to rebuild the parts is also large.  There are also costs to operate and maintain the stations.

Some rough estimates can be made by what should be known.  Unfortunately, the numbers provided to the public by NJT do not seem to mesh with the Chase Management dinky ridership survey.  I'm awaiting clarification from NJT, hence the calculations here are null (xxx).  The dinky provides about xxx one way rides per year and the fare is $2.75 per ride.  It costs NJT $1.8 million dollars per year to provide the service.  NJT has stated on several occasions that the farebox recovery pays 54% of the cost.  Therefore the cost is about $xxx per ride.  The dinky run is 2.7 miles long.  That works out to about $xxx per passenger mile.

By contrast, the university's Tiger Transit bus system, where there are no fares and the public can ride for free, provides about 600,000 rides per year.  They won't release the numbers, but publicly available data shows that they spent $3.08 million with the company that provides the busses and the drivers in 2010.  There are of course some administrative costs inside the university related to that system as well as costs for signage, IT cost for location information and other overheads.  A good guess of system operating cost would be $4.0 million per year.  Unfortunately, the average trip length data does not exist; a good starting guess might be 1 mile.  That works out to $6.66 per passenger mile or $6.66 per ride.  Of course, there are no guideway costs included because someone else is covering that expense.  The Tiger Transit system could ultimately be replaced by PRT, but only when an on campus network and elements of the 30 year plan are in place.

For PRT there doesn't seem to be any good cost data from the field that has been released but it may well come soon.  Estimates from experts in the field place PRT operating costs in the range of $0.40 per passenger mile. We do know as stated above that the energy cost per passenger mile for PRT is significantly better than other modes.  Since the PRT passengers are the "operators" there is no operator cost.

Any transportation option needs to be able to attract riders, which essentially translates into getting people out of their automobiles.  Improvements in rail service are capable of producing several percentage points increase in this regard.  To really get people out of their cars requires an alternative that approximates the same advantages of the automobile.  The most salient one is the fact that PRT runs on demand, just like the automobile.  Another is the option to not share the vehicle with strangers.  Add to that the advantage that the chore and cost finding parking is eliminated.  All these factors make PRT very attractive provided a suitable service network is in place.  Claims have been made that PRT can increase ridership 25% to 60%.  This estimate is overly optimistic for Princeton and there are no comparable examples to examine.

If you have made it through the previous discussion is should be painfully obvious why PRT is best for our future.

History of the SPURTS Idea:

SPURTS is the work of Princeton resident and local businessman Chip Crider, myself.  Here is how it happened:

In early 2007, when Princeton University first presented the concept for their new arts district, a few citizens objected to their plan to move the Dinky about 500 feet farther away from downtown.  Surely we could do better than staunchly defending the status quo.  After all, the Dinky is state of the art for 1895.  I started thinking about this, examining possible solutions and seeking new opportunities.  Since I arrived here in 1973 for Graduate School I have always lived on the west side of town.  I've seen thousands trudging up University Place toward downtown with their baggage in tow.  Obviously, the Dinky is in the wrong place -- it needs to be closer to downtown, and commuter parking for it should be outside of town at a separate stop.  I could see that it was possible to get the Dinky closer to downtown, but that this solution would be clumsy and awkward, due to the size of the train and the huge turning radius required.  So I postulated a new system that was more nimble, with a smaller turning radius and the ability to change elevation quickly.  That was when I found that a right of way to service our needs could be achieved if we acted soon.  Then, when I went looking for trains, I found PRT.

I've had numerous discussions with people around town.  Many have offered suggestions and, because of those, my ideas have evolved.

It became clear early that Princeton University is the only local entity that could make such a plan work.  After communications with former mayor Marvin Reed, I was able to make a brief presentation of my plan to the Planning Board Master Plan Update Committee in mid 2007.  This was immediately following a NJT presentation on their Bus Rapid Transit idea.  After the meeting I was able to become acquainted with Princeton University VP Bob Durkee.  Through his gracious efforts several meetings were set up with all the appropriate university officials.  They appeared receptive and interested.  I never received a formal reply to my challenges but an article in the Daily Princetonian (Oct. 20, 2008) read:

In an e-mail, University Vice President and Secretary Bob Durkee '69 said that he "remain[s] intrigued by what [Crider] has proposed," but "can't yet assess the feasibility of the plan." "I believe there is nothing we are proposing in our campus plan that would preclude implementing a plan like SPURTS in the future"....

I know of no further action being taken by the university.  And then the economy collapsed.  I can't fault them for inaction at that time (fall, 2008).  After all, if your house is on fire you don't go out and start work on your new addition.  But now, that the fire is under control, it is time to start moving ahead again.

Occasionally someone asks why I am doing this.  It is certainly not for money.  I've spent about $3k and thousands of hours and so far have gotten 2 free lunches; doesn't look like gold mine to me.  My business is building scientific instruments and if you should happen to notice any connections between electron diffraction in ultrahigh vacuum and PRT please, please let me know.  In all seriousness, this is my civic contribution.  I'm not a politician and could never be one.  Over my time in town I've seen a dearth of transportation planning and the most significant idea (the Princeton bypass) defeated by the NIMBY's and by other towns.  Transportation is not very glamorous and the only time people talk about it is when it doesn't work.  I simply saw something that needed to be done and am doing it.

I am indebted to my wife who not only did the graphics for this site but who also keeps me on track.  Arch Davis, former resident traffic guru has also helped me greatly.  I also acknowledge help from Boro Administrator Bob Bruschi, as well as discussions with Sheldon Sturges of Princeton Future, Carlos Rodrigues at Regional Plan Association, Leo Arons, other residents, politicians, planners, engineers, numerous local architects and many professionals at the University.

Ideas, observations, and solutions from Chip Crider, PhD '79.  I welcome your comments at the address below.

Please click on the dots within the partial e-mail address below to reveal the full address:


You will be directed to reCAPTCHA Mailhide, a service that protects e-mail addresses from spammers.  Type in the letters shown, and then select and copy the full e-mail address.  Or, you can simply click on the address in the window to open your default e-mail application.

This page last updated on May 10, 2012